Otters Face a Hidden Parasite Burden, and Most Species Have Never Been Surveyed

Otters occupy a precarious ecological position. As semi-aquatic members of the mustelid subfamily Lutrinae, they forage in rivers, lakes, and coastal waters while denning and resting on land, which means their immune systems are challenged by pathogens drawn from two entirely different worlds. Twelve of the fourteen recognized otter species are currently listed by the IUCN as near-threatened, vulnerable, or endangered, and a new systematic review published in Frontiers in Mammal Science suggests that parasitic diseases, long an overlooked stressor, may be compounding the pressures of habitat loss, fragmentation, climate change, and increasing contact with humans.

The review, conducted by Stacy R. Cotey of Michigan Technological University and Mason V. Reichard of Oklahoma State University’s College of Veterinary Medicine, followed PRISMA guidelines and searched Google Scholar, Web of Science, and the ProQuest Dissertation and Theses databases using the terms otter plus parasite and otter plus disease. The initial sweep returned 4,515 sources. After duplicates were removed, titles and abstracts were screened by both researchers, and full papers were read in their entirety, the final dataset rested on 240 papers, the earliest of which dates to 1924. Publications have climbed steadily each decade, with 76 papers appearing between 2010 and 2019 and the 2020s on track to match or exceed that figure.

The headline numbers are striking. Across ten otter species, the authors cataloged 146 parasite genera representing 164 species, distributed across five major phyla: the apicomplexan protozoa, thorny-headed worms (Acanthocephala), roundworms (Nematoda), flatworms (Platyhelminthes), and arthropods such as ticks, mites, and insects, plus a residual category of tick-borne bacteria and other parasites spanning nine additional phyla. For each parasite-otter pairing with sufficient data, the team calculated prevalence and a 95 percent confidence interval using a modified Wald method for proportions.

Yet the coverage is profoundly lopsided. The Eurasian otter (Lutra lutra) dominated the literature with 89 papers and the highest parasite richness at 184 species, followed by the sea otter (Enhydra lutris) with 79 papers and 41 parasite species, and the North American river otter (Lontra canadensis) with 47 papers and 66 species. Three species, the smooth-coated otter (Lutrogale perspicillata), the hairy-nosed otter (Lutra sumatrana), and the Congo clawless otter (Aonyx congicus), have no published parasite studies at all in the databases searched. Seven additional otter species had fewer than 15 papers each. The newly recognized Neotropical species Lontra annectens, split from Lontra longicaudis on the basis of genome-wide data, has not yet been evaluated by the IUCN and has no parasite records of its own.

Among the protozoa, three parasites stand out as highly pathogenic: Toxoplasma gondii, Sarcocystis neurona, and Neospora caninum. Otters are accidental hosts of all three, and infections can be fatal. Toxoplasma gondii was the most heavily sampled apicomplexan, detected in five otter species, with prevalence of 12.6 percent in 5,307 sea otters, 14.3 percent in 1,008 Eurasian otters, 29.2 percent in 489 North American river otters, and 76.9 percent in a small sample of 13 southern river otters. Sarcocystis neurona, which together with T. gondii causes encephalitis and mortality in sea otters, showed an overall prevalence of 18.5 percent across 1,612 sea otters, and 5.1 percent of 1,350 sampled sea otters carried co-infections of both parasites. Neospora caninum reached 19.4 percent in 278 sea otters. The review also flags the considerable genetic diversity of T. gondii as a concern, because recombination among strains could generate more virulent variants with population-level consequences.

The helminths tell a story shaped by diet. Nematodes were the most species-rich group, with 58 genera and 58 species, a diversity the authors attribute largely to the otters’ piscivorous feeding on fish, amphibians, and aquatic invertebrates. Several roundworms are otter specialists, including Dirofilaria lutrae (16.5 percent prevalence in North American river otters), Dracunculus lutrae (87.7 percent in the same host), Skrjabingylus lutrae, and Strongyloides lutrae. Highly pathogenic generalists such as Ancylostoma species, the mustelid specialist Skrjabingylus nasicola, Dirofilaria immitis, and Trichinella species also appear in the record. Flatworms, with 47 genera and 52 species, are likewise acquired through prey: Diphyllobothrium species use fish as intermediate hosts, Paragonimus kellicotti cycles through crayfish and causes lesions in the bile ducts and gall bladder, and both carry zoonotic potential. Notably, warmer temperatures, reduced rainfall, and fewer frost days have been linked to increased prevalence of the biliary trematodes Pseudamphistomum truncatum and Metorchis albidus in Eurasian otters, a direct climate signal.

The acanthocephalans, though fewer in number with 6 genera and 10 species, include one of the review’s most consequential findings. Corynosoma enhydri, a specialist of the sea otter, showed an overall prevalence of 84.4 percent across 404 otters sampled in five studies. Recent work cited in the review found that this thorny-headed worm parasitizes sea otter pups at an early age, potentially suppressing their immune response and increasing susceptibility to other parasites or bacterial infections that led to mortalities. Heavy infestations of Profilicollis altmani and P. kenti have directly caused sea otter deaths. Arthropods, meanwhile, include the nasal mite Halarachne halichoteri, which causes respiratory disease and reached 25.6 percent prevalence in 156 sea otters, and the New World screwworm fly Cochliomyia hominivorax, whose tissue-eating larvae were found in a single giant otter. Ticks are relatively common on otters because several species survive prolonged submersion: Dermacentor reticulatus adults can survive up to 65 days underwater, Ixodes ricinus up to 44 days, and Amblyomma americanum up to 70 days, all three of which naturally infest otters and can vector pathogens such as Babesia, Anaplasma, and Rickettsia.

The review also documents parasites moving between humans, domestic animals, and wildlife in both directions. Giardia species were the most commonly reported of the tick-borne and miscellaneous group, detected in four otter species with prevalence ranging from 1 percent in sea otters to 20.8 percent in giant otters, and co-infections of Giardia and Cryptosporidium were found in Neotropical and giant otters. Agricultural runoff and untreated sewage are identified as routes by which these protozoa from domestic animals and humans enter aquatic systems. Growing support exists for the reverse hypothesis as well: Giardia duodenalis from human wastewater may infect aquatic mammals such as otters, which then serve as additional hosts for the parasite. Leishmania infantum, Encephalitozoon cuniculi, and the tick-borne bacterium Ehrlichia canis round out the list of highly pathogenic species recorded in otters.

The authors are candid about the biases threading through their dataset. Most studies were conducted in Europe and North America even though parasite diversity is higher in subtropical and tropical regions, a pattern reflecting publication bias toward English-language journals, limited access to study areas, and funding constraints. Prevalence estimates are further distorted by diagnostic method, since antibody tests detect previous infections while dissection and DNA methods detect active ones, and by the fact that most surveys sampled sick or dead animals, skewing prevalence upward relative to systematic surveillance and preferentially detecting parasites that cause visible symptoms. Opportunistic carcass sampling also biases the population toward dispersing young and toward sites near human development, where additional stressors impair otter immunity. Small sample sizes, sometimes fewer than ten animals, and repeated testing of the same otters for multiple parasites compound the uncertainty. The review notes that DNA amplification through PCR has broadened surveillance to environmental DNA and feces, but it has also revealed organisms such as the free-living soil nematodes Aphelenchus avenae, Diploscapter species, and Prionchulus species that otters likely ingested accidentally with prey rather than hosting as true parasites.

The broader ecological logic of the review is that generalist parasites, which infect a wide range of mammalian or vertebrate hosts, pose the greatest threat to vulnerable species because they can persist in tolerant hosts across broad ranges, continually exposing sensitive hosts to virulent stages. Fifteen of the cataloged parasites are specific to otters and eight to mustelids, but the majority, particularly the highly pathogenic ones, are generalists. Environmental degradation adds another layer: parasites with complex multi-host life cycles tend to decline in stressed environments, while single-host, monoxenous parasites such as Eimeria, Giardia, Cryptosporidium, and intestinal nematodes including Ascaris, Toxocara, and Trichuris, all of which have been found in otters, may thrive, especially when stress makes hosts more susceptible. The authors conclude that parasites must be treated as a factor when evaluating otter populations and designing conservation plans, and that systematic surveillance, detailed spatial data, and basic information on host sex, age, and behavior are the missing foundations for protecting twelve of the world’s fourteen otter species from a stressor that science has barely begun to measure.

Subject of Research: A systematic review found 164 parasite species reported in ten otter species, with no parasite studies for three threatened otters and generalist pathogens posing the greatest conservation risk.

Article Title: A systematic review of parasitic diseases of otters

Article References: Cotey, S. R., & Reichard, M. V. (2025). A systematic review of parasitic diseases of otters. Frontiers in Mammal Science, 4, Article 1673488. https://doi.org/10.3389/fmamm.2025.1673488

Image Credits: AI Generated

DOI: 10.3389/fmamm.2025.1673488

Keywords: otters, Lutrinae, parasites, Toxoplasma gondii, Sarcocystis neurona, sea otter, Eurasian otter, North American river otter, systematic review, wildlife disease, conservation, Nematoda